Investigations of neural and/or muscular problems associated with impaired movement of the limbs, and of how coordinated motor behavior is planned and controlled by the central nervous system (CNS), ideally require the ability to monitor the movement of one or more joints of a limb under various mechanical conditions. A number of devices have been proposed for manipulating limbs under various conditions of load, but most of such devices are concerned with end-point loading of the limb. For example, U.S. Pat. No. 5,210,772, issued Apr. 13, 1993 to Maxwell, relates to a complex linkage which attaches to a subject's limb at a single point, and provides forces to resist limb movement. U.S. Pat. No. 5,466,213, issued Nov. 14, 1995 to Hogan et al. describes a robotic therapist consisting of a computer-controlled mechanical linkage that interfaces with a subject's hand and guides the arm through a range of movement. U.S. Pat. No. 5,830,160, issued Nov. 3, 1998 to Reinkensmeyer relates to a system consisting of a guide that permits limb movement along a linear path. Forces can be applied to resist or aid movement of the limb along the path. While these systems may be useful in a rehabilitation program for individuals with impaired movement of limbs, they are not readily able to provide information related to the mechanics of limb movement.
When an individual suffers brain injury from stroke, trauma, or the like, there often results decreased control of one or more limbs. This decreased control is associated with a partial loss of the central nervous system's ability to coordinate motor patterns of muscles at various joints of the limb to smoothly move the limb in space. Effective treatment of such disabilities can be enhanced with detailed knowledge of the mechanics of limb movements; however, none of the instruments proposed to date are capable of providing the required data.
In addition to providing effective treatment to individuals with motor disabilities, it is desirable to gain a further understanding of neurological processes taking place in the CNS. It is generally agreed that limb movements provide an ideal paradigm for understanding how sensory information is converted into coordinated motor behavior (Soechting et al., 1992). In particular, visual-guided reaching movements has become an important paradigm to study how regions of the brain, such as primary motor cortex (MI), are involved in the planning and control of voluntary movement (Caminiti et al., 1990; Georgopoulos, 1995; Kalaska et al., 1992; Shen et al., 1997). Not surprisingly, studies with non-human primates indicate that neural activity related to a specific feature of movement covaries with many other movement variables. To dissociate the various parameters of movement, several studies have examined the response patterns of individual cells when reaching movements are performed under different mechanical loads or arm posture (Caminiti et al., 1990; Kalaska et al., 1989; Scott et al., 1997). A consistent finding from these studies is that the directional tuning of many MI cells is modified by load or posture. These results suggest that the activity of cells is not simply related to the direction of hand movement, but may be more related to intrinsic features of the task, such as the joint kinematics or kinetics. However, further progress on interpreting the nature of the discharge patterns of these cells based on features of motor execution are difficult in these studies because neural activity was only related to hand motion.
Thus, the major stumbling block for identifying the nature of the neural representation in MI during reaching movements is the difficulty in directly quantifying and manipulating the mechanics of multi-joint motion. While a number of devices have been proposed to apply loads to the arm during multi-joint movements (Gomi et al., 1996; Shadmehr et al., 1994), these devices are of limited use because the loads are applied through the band.